In the manufacture of hydraulic cement, the normal way that uses lime has the drawback of emitting large quantities of CO2 into the atmosphere and consuming large quantities of energy in the form of fuel.
Other materials with a high calcium content may be used for producing hydraulic binders. Among them, slag from the metallurgical industry is a source of raw material that can be reprocessed by manufacturing hydraulic cement, thus avoiding the aforementioned drawbacks. The use of this slag for manufacturing cement also makes it possible to dispose of this waste from the metallurgical industries.
However, during production thereof, the slag give rise to metal particles of various sizes, the presence of which in the cement may degrade the performance thereof, in particular if coarse particles remain in the product. When the grinder used for producing cement is a ball mill, the metal particles are fragmented during the grinding process by the impacts between the balls.
Because of their small granulometry in the end product their presence does not impair the quality of the cement. However, ball mills have a low energy efficiency compared with other grinders operating by compressing a bed of material, and consumes approximately twice the quantity of energy required by the latter.
One example of a grinder operating by material bed compression is disclosed in the document EP 0486371. The grinder in this document comprises a circular track formed by an internal wall of a cylinder, roughly horizontal, driven in a rotation movement, and a roller able to roll on the track.
When the cylinder is set in rotation, the material is ground, compressed between the roller and the internal wall of the cylinder.
However, compression grinding does not completely fragment the metal particles because of their high ductility. Coarse metal particles therefore remain present in the material leaving the grinder, or accumulate in the grinder itself.
A magnetic separation step is generally present at the input of the grinder. The role of this step is to prevent the entry of large magnetic elements that may damage the wheels (rollers) of the grinders by compression of material.
However, this magnetic separation step is not capable of eliminating the metal fractions that are included in pieces of slag.
There is also known from the document WO 97/14760 an abrasive product consisting mainly of iron oxide, with a granulometry of between 100 μm and 600 μm, its use, its production method and a method of producing secondary metal granular products from slag. The purpose of this method is to obtain secondary metal granular products that find a particular application for example as an abrasive additive or for producing ballast.
This method aims to select the majority metal part of a slag and comprises successively the following steps:                feeding the slag into a grinder,        grinding the slag in the grinder, obtaining a ground material,        sorting the ground material under blown air in order to separate a mainly metal fraction from a first fine fraction,        magnetically sorting said mainly metal fraction in order to separate the non-magnetic products from a refined product,        sorting the refined product under blown air in order to separate metal granules from a second fine fraction, the granules comprising a plurality of distinct sizes.        
Such a method makes it possible to select the majority metal part of a material but would not be suitable for satisfactorily extracting metal impurities contained residually in a mineral material.